1. Field of the Invention
The present invention relates to an OFDM signal generation method, and apparatus, for generating a signal subjected to Orthogonal Frequency Division Multiplexing hereafter referred to as OFDM) modulation, and in particular to a technique suitable for the case where a transmission system is tested by transmitting an OFDM signal.
2. Description of the Related Art
Heretofore, as one of modulation systems in the case where a comparatively large amount of digital data are sent by radio transmission or the like, an OFDM modulation has been put to practical use. The OFDM modulation system is a system for transmitting transmission data as multi-carrier data distributed into a plurality of sub-carriers. By using the OFDM modulation system, radio transmission of a large amount of data can be conducted efficiently.
When testing a receiving apparatus for receiving such an OFDM signal, an appropriate random bit sequence was inputted to a transmission data input portion of a transmitting apparatus for transmitting an OFDM signal. An OFDM modulated signal was thus generated. The receiving apparatus was made to receive a signal transmitted from this transmitting apparatus for the purpose of testing. Also when testing a transmitting system of a communication terminal having a function of transmitting a OFDM modulated signal, data of an appropriate random bit sequence were inputted to the communication terminal and thus an OFDM modulated signal was generated.
If the above described random bit sequence for testing was inputted to an inverse fast Fourier transform (IFFT) circuit serving as an OFDM signal generation means to generate an OFDM signal, the generated OFDM modulated signal looked like just a random noise when viewed in the time axis direction. Even when viewed in the frequency axis direction, it was difficult to determine whether an OFDM modulated signal was outputted correctly. It was thus difficult to determine from the conventional states of test signals whether or not a correct processing was conducted. FIG. 1 is a diagram showing an example of a conventional test signal. In FIG. 1, FIG. 1A is a diagram showing a waveform when viewed in the frequency axis direction, while FIG. 1B, is a diagram showing a baseband waveform when viewed in the time axis direction. It is difficult to grasp the state from only these waveforms.
Furthermore, when a test is conducted by inputting an OFDM modulated signal to a receiving apparatus, it was especially difficult to determine whether or not a fast Fourier transform (FFT) circuit for transforming the frequency axis of an OFDM signal to its time axis was functioning normally.
In addition, in the case where an OFDM modulated signal was generated by inputting a random bit sequence to the IFFT circuit, there was a possibility that phases of sub-carriers were aligned if processing was conducted without considering phases of the sub-carriers. When the waveform was observed on the time axis in such a case, an extremely large peak was observed and the signal was distorted due to exceeding the dynamic range of a modulator. FIG. 2 shows an example of such a case. In FIG. 2, FIG. 2A is a diagram showing a waveform when viewed in the frequency axis direction. FIG. 2B is a diagram showing a baseband waveform when viewed in the time axis direction. Even if there is no abnormality when viewed in the frequency axis direction, an extremely large peak waveform might appear when viewed in the time axis direction. Such an abnormal state is not desirable in testing.
An object of the present invention is to make it possible to favorably test the transmitting apparatus and the receiving apparatus using an OFDM modulated signal.
In order to solve this problem, an OFDM signal generation method of the present invention includes the steps of converting time axis representation of predetermined input data to frequency axis representation thereof at every predetermined unit by using the inverse Fourier transform and thereby generating an OFDM signal using a plurality of sub-carriers; and stopping output of an arbitrary sub-carrier included in a plurality of sub-carriers forming the OFDM signal in response to a predetermined command.
According to the OFDM signal generation method of the present invention, an OFDM signal having an arbitrary number of sub-carriers can be generated.
Furthermore, an OFDM signal generation apparatus of the present invention includes a conversion means for converting time axis representation of predetermined input data to frequency axis representation thereof at every predetermined unit by using the inverse Fourier transform, thereby generating an OFDM signal using a plurality of sub-carriers; and a control means for stopping output of an arbitrary sub-carrier included in a plurality of sub-carriers forming the OFDM signal to be converted by the conversion means , in response to a predetermined command.
According to the OFDM signal generation apparatus of the present invention, an OFDM signal having an arbitrary number of sub-carriers can be generated by the conversion means on the basis of a command given to the control means.